Previous work suggested the existence of an opiate receptor complex, consisting of adjacent and interacting mu-and delta-binding sites. This hypothesis was based on the finding of apparent noncompetitive interactions between mu and delta binding sites. This project focused on a more detailed study of apparent noncompetitive interactions. Using rat brain membranes, and in vitro ligand binding methods, the interaction of mu ligands with 3H-D-ala2-D-leu5-enkephalin binding to delta receptors was analyzed with computer curve fitting methods. The results showed that 3H-D-ala2-D-leu5-enkephalin labeled two binding sites, and that mu ligands were competitive inhibitors at the higher affinity binding site, and noncompetitive inhibitors at the lower affinity binding site. Similarly, it was shown that the opiate antagonist, 3H-naloxone labeled two binding sites, mu and kappa receptors, and the leucine-enkephalin was a noncompetitive inhibitor at the mu binding site, but a competitive inhibitor at the kappa binding site. These results support the existence of an opiate receptor complex consisting of interacting mu and delta binding sites. Future work will explore several issues, including the existence of a kappa binding site in the receptor complex, as well as the physiological function of this receptor in light of the many chemical signals generated by several molecular forms of opiate peptides which coexist and are presumably coreleased from enkephalinergic terminals.